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US Senator for Vermont Bernie Sanders has kicked off his 2020 presidential campaign with a forceful denunciation of Donald Trump, who he called "the most dangerous president in modern American history."

At the rally in his birth place of Brooklyn, New York, Sanders set himself up as the anti-Trump.

"I want to welcome you to a campaign which says loudly and clearly that the underlying principles of our government will not be greed, hatred and lies...It will not be racism, sexism, xenophobia, homophobia and religious bigotry."

Sanders' 2020 manifesto includes healthcare for all, a $15-an-hour minimum wage and action on climate change.

It's the second run at the presidency for the populist progressive - after he lost the Democratic nomination to establishment favourite Hillary Clinton in 2016.

That campaign was fueled by passionate grass roots support among young liberals -- something he plans to recreate with a million active volunteers in each of the country's 50 states.

Last time around, he was an outsider -- he's now a front runner, facing multiple formidable rivals.

Sanders is now headed to Selma, Alabama to mark the anniversary of the 1965 civil rights clash known as Bloody Sunday.